Went to the range last weekend to break in/sight in a Bushy A3 flat top .308.  The rifle shot well except for a sight problem.  I bought DPMS iron sights for it, part numbers FS-DET-S and FT-RS.  Firing on the 300" range, I couldn't get the sights to depress any lower than 4" abvove the aim point.  I was getting good tight groups, so I don't think it was my marksmanship.  The front sight has an adjustable post, and it was fully raised.  The rears were fully depressed to lower the elevation.  I called DPMS before purchase, and they sid the sights would work, since the DPMS and Bushy flat tops had identical elevation differences in the gas block mount and the rear rail.  I called again today, and they suggested a taller post, but it would only elevate the post by .030", which doesn't  seem enough to adjust out a 4" problem.  Any suggestions?  I've been shooting for nearly 50 years, but I'm new to AR type rifles, except for a very brief period in Nam when the Marine Corps suggested I try one.  The one I got was a POS, so I went back to my M-14.
All suggestions appreciated.